1. Field of the Invention
The present invention relates to a liquid crystal display apparatus, and particularly to a liquid crystal display apparatus using a two-terminal nonlinear resistance device.
2. Description of the Related Art
Portable OA equipment such as personal computers have been considerably developed in recent years. Accordingly, the cutting-down of production costs and reduction of power consumption of liquid crystal display (LCD) apparatuses used for such OA equipment have become important challenges.
Among various types of methods for driving such LCD apparatus which are currently used, a simple matrix type is the most promising for cutting down the production costs. Recently, multiple information media have been developed, and a display with multiple gray levels (a full-color display, a multi-color display) has been essentially demanded. However, the simple matrix type has difficulty in realizing the display with multiple gray levels. Accordingly, a method in which each pixel is provided with an active device (a switching device) is proposed.
A typical active device is a thin film transistor (TFT). However, during the production process, a TFT requires a large number of masks to be used in photolithography, so the production yield is decreased and the production cost is disadvantageously increased. On the other hand, a thin film diode (TFD) which is one type of active device only requires about three masks to be used in the production process, so that the price of the device can be lowered. Among such TFDs currently used, a metal-insulator-metal (MIM) device having a nonlinear resistive layer of tantalum oxide (Ta.sub.2 O.sub.5) is widely used.
FIG. 12A shows the general structure of one pixel in a conventional LCD apparatus with an MIM device. FIG. 12B shows the cross section taken along the line a--a in FIG. 12A. Such a conventional structure is described, for example, in "Liquid Crystal Display", Sharp Corp., Liquid Crystal Display Division, p. 71, and Japanese Patent Publication No. 1-35352. The MIM device includes, on a glass substrate 31, a lower electrode 32 of tantalum (Ta) or the like, a nonlinear resistive layer 33 of tantalum oxide (Ta.sub.2 O.sub.5) or the like, and an upper electrode 34 of chromium (Cr) or the like, in this order from the substrate 31. The nonlinear resistive layer 33 is often formed by anodization of the lower electrode 32.
The upper electrode 34 of the MIM device having the above-described structure has end portions which are overlapped by a part of pixel electrode 35, so as to be connected to the pixel electrode 35. The pixel electrode 35 is formed from a transparent conductive film of indium tin oxide (ITO) or the like.
However, the conventional MIM device having the nonlinear resistive layer of Ta.sub.2 O.sub.5 has the following problems. First, the current-to-voltage (I--V) characteristic as a two-terminal device is not sufficiently steep, i.e., the ON/OFF ratio is not high enough. In general, in the MIM device having the nonlinear resistive layer of Ta.sub.2 O.sub.5, I.sub.20V /I.sub.5V is nearly equal to about 10.sup.3, where I.sub.20V and I.sub.5V are current values when an ON voltage of 20 V and an OFF voltage of 5 V are applied, respectively. It is difficult to achieve multiple gray levels by using an MIM device having such an ON/OFF ratio.
Secondly, the I--V characteristic of the conventional MIM device exhibits poor symmetrical property with respect to the polarity of the applied voltage. The nonlinear resistive layer of the MIM device is formed by anodizing the lower electrode, as described above. On the other hand, the upper electrode is often deposited by sputtering on the nonlinear resistive layer. Because the formation methods are different, the condition at the interface between the nonlinear resistive layer and the lower electrode is different from the condition at the interface between the nonlinear resistive layer and the upper electrode. This difference in condition may result in asymmetry of current conductivity. In the case where the material for the upper electrode is different from the material for the lower electrode, the asymmetry of the I--V characteristic can more significantly appear. Such electric asymmetry may cause a bias voltage to be applied to liquid crystal when the liquid crystal is driven, so that there arises a problem in that flickering may occur.
In order to eliminate such electrical asymmetry, a so-called ring structure and a back-to-back structure in which a plurality of conventional nonlinear devices are combined are proposed (see Japanese Laid-Open Patent Publication Nos. 2-308227 and 4-73716, and the like). However, in such structures, since each nonlinear resistance device is an MIM device having a nonlinear resistive layer of Ta.sub.2 O.sub.5, it is impossible to achieve sufficient steepness in the I--V characteristic.
In addition, in the structure of the conventional MIM device shown in FIG. 12A, the upper electrode and the lower electrode are crossed, so that the nonlinear resistive layer includes a stepped portion in a region functioning as an active device. In such stepped portion, leakage current may easily occur.